1. Field of the Invention
The present invention relates generally to a device and method for simulating and analyzing transmitted physical force waves in diverse media, said media being comparable with various biological fluids and said device allowing for the control and manipulation of the various internal and external constraints controllably applied to that media.
2. Description of the Related Art
Biologic and physiologic systems for mammals have two circulations—the vascular and the lymphatic. At the terminal loops of these two systems we have all elements in close proximity, microscopically close. Previous physiologic studies of this terminal bed at microscopic level essentially just show summary effects, such as flow variations—and then, only in the vascular, red-cell-filled portion of the circulation. Histological exam of this terminal set of circulatory loops clearly shows the vascular, red-cell loops to be continuous from afferent to efferent but the lymphatics start as blunt end channels. Fluid exchange between them is ubiquitous but it is known from clinical and experimental evidence that if the pulsatile nature of arterial flow is dampened or lost, then lymphatic flow stops. The exact nature of this relationship is so far minimally investigated. It is extremely challenging to get in vivo, invasive or non-invasive, instrumentation into these systems at microscopic levels and to sort out the intertwined signals and messages of this complex network of vessels and pulses and flows. Because of that, we propose a “bench-top” simulation for meaningful analysis of the elements and components of this biological system in isolation, allowing controlled variation and study of the potential relationships with the long term vision of being able to re-apply them in vivo.
An average human peripheral pulse pressure at the level of a major named artery is about 110-120/70-80 mmHg giving a pulse pressure of 40 Torr and a mean pressure of 90-100 Torr. This pressure wave decreases as we move peripherally and the mean pressure decreases to approximately 32 Torr as the afferent arteriole enters the terminal capillary loop where it filters extracellular and interstitial fluid as it passes through this capillary loop resistance and the pressure is generally a 20 mmHg drop across the loop and average pressure at the beginning of the efferent venous channel is 12 mmHg with no or a minimally detectable pulse differential. At this level, the blunt end of the lymphatic capillary is collecting this filtrate and starting its flow at a pressure of roughly 7 mmHg. Downhill gradients therefore exist in this system. However, we know that without a pulsating arterial system, flow ceases or is very ineffective in this early lymphatic collection.
Most of these interactions take place in an environment fluidically that is very complex, consisting of liquids of various oncologic and osmotic pressures and concentrations, some with and some without cells, inside of these circulatory vessels or channels. And these vessels, then, are in direct apposition to, in fact they are bathed in or embedded in, an environment of a visco-elastic matrix of fiber and cells suffused in its interstices with this interstitial and extra-cellular fluid. For simplicity and the ability to analyze components, we will refer to the intra-vessel component as “liquid” and the material outside the vessels as “gel.”
Two areas of note in the body have a different fluid distribution. The first is the chest or thorax portion of the body with its ribbed, buttressed walls and its muscular diaphragm able to generate its own separate set of pressure and thus inspire and expire air. So this section of the body interfaces gas with these liquids and gels. The other is bone where we have a porous solid filled with liquids. Our device interfaces with all these states of fluidity.
The human body is highly compartmentalized and specialized. If we look at the body in all its complexity, its different tissue types and organs, its functional “systems,” its solid and hollow viscera, etc., the permutations seem almost unlimited. But based on repeating patterns of compartments, there are actually just a few types that we need to consider. If we look at the extremities, for example, we see the outer layer of skin that we will refer to as the “external constraint.” Inside this we have several clearly identifiable major chambers of strong, more fibrous, less yielding walls called fascia. We reproducibly, predictably have the deep fascia which parallels the skin and separates the “superficial” and “deep” compartments of the extremity. This compartment layer forms a circumferential layer around all the extremities and the trunk of the body. Inside this layer of deep fascia in the extremities, we have one or two bones centrally and strong separating layers called “intermuscular septi” that join the bone to the deep fascia creating two or three compartments depending on the location. Inside each of these compartments, then, we have additional layers, less strong or thick but still less elastic than the skin, that surround and contain or compartmentalize the muscle groups and the neurocirculatory bundles—which include the larger lymph collecting channels. We will refer to all of these internal, fascial, compartment-producing layers as “internal constraints.”
If we look at the thorax or chest, we have gas against the circulatory channels and a more rigid external wall. In the abdomen then we have relatively thin hollow tubes, the intestines, filled with liquid. This is a much different liquid to gel ratio than we find in the extremities. And in the abdomen we have larger more distinct circulatory channels not in compartments but in thin membranes (mesentery) in essentially direct contact with the viscera around them and with the large aortic pulse behind them.
Considering how these elements are arranged and manipulated to achieve special circumstances in circulation, multiple examples are found. Based on the premise that a pressure wave is necessary for (lymphatic) flow, we see this arranged in a variety of patterns: first, for example, with arterial pulsation intimately intertwined with the “two circulations” in peripheral and terminal vascular beds; then we also see it in enclosed neuro-vascular compartments (internal constraints) in the extremities along the paths of major longitudinally arrayed vessels. We see it combined in compartments with different types of pressure waves: muscular action produces a compressive wave in the muscle compartments of the extremity; the slow undulating wave of peristalsis, or abdominal hollow visceral contraction, is yet another type of pressure wave that the vessels are exposed to and compartmentalized with. One of the most dramatic, and applicable examples for our purposes, is the inter-species mammalian differences between, on the one hand, a human forearm, in which the skin is thin, supple and elastic, and in an individual with low body fat, allows us to see the veins bulging out, pushing the skin with just 10 Torr, and then the tissue tenting inward (at transiently created negative pressure because of the change in surface area and volume in relation to ambient air pressure) between the outward protruding pairs of flexed tendons, and, on the other hand, the thick, non-flexible or minimally elastic skin of the giraffe's ankle which aids in the return of blood up a significant height back to the right atrium of the heart and aids in maintenance of the correct filtration pressures given the size of the heart pump in the giraffe which is twice the size of mammals of similar weight.
These examples of the variation of external constraint have direct clinical application and constitute requirements for the construction of our device. We want to be able to manipulate the external constraints on our device to mimic at least the four major types of clinically applied external dressings or garments that are felt to have vascular (flow) effects: 1) rigid, immobile, closely-applied dressing (at least non-elastic if not rigid), e.g. “contact casting” for diabetic foot wounds, orthopedic casting for trauma, Unna boot for stasis ulcers; 2) flexible, elastic single or multilayer wrap, e.g. Jones wrap, 2 or 3-layer compression wraps for stasis disease, ace wrap for athletic injuries; 3) air or water filled compression wraps (positive pressure), e.g. “G suits,” MAST trousers for trauma, compression garments for venous stasis disease; and 4) vacuum-press applied dressings (negative pressure), e.g. compressed-foam suction applied external dressings such as the VAC, or external suction dressings with other materials.
The effects that changing volume and surface area inside a space can have on fluid dynamics and vice-versa are well known. Ideally, a device to duplicate physiologic states should have this ability to change volumes by manipulating the external constraints and changing the internal pressures independently.
An area of wound-dressing and wound-healing technology is known as “moist wound healing.” Three major components that constitute the external and physical environment of the healing wound should, in an ideal wound-healing environment, be controlled. First, wound healing is inversely related to bacterial growth. Second, it has been shown that, holding other variables constant, there is a clear linear relationship between the moisture level at the wound-site and the rate of epithelial advancement. The final important characteristic is the surface contact property of the wound dressing. The surface contact property can help to control the other two major factors, but must be made of a suitable material that promotes endurance of the dressing as well as comfort to the patient.
Thin pieces of foam have been used in moist-wound healing applications. The external face of the thin foam was more open allowing for enough moisture retention initially, but then allowing drying to occur with the dressing still in place. Because this foam did not adhere to the wound, it could be moved or removed without disrupting the epithelium. However, this practice was often limited to use with incisions smaller than one inch, as the thin foam is incapable of managing a large amount of exudates for a large, fresh wound, and if exudates accumulate under the foam piece the foam will lose surface contact, which allows bacteria to build up.
In general, epithelium advances or migrates best if moisture is maximized and then matures best if moisture is minimized. Although the idea of moist wound healing is not new, it is still evolving.
Another important aspect of wound healing relates to the respective roles of the vascular and lymphatic circulatory systems, both of which are involved in wound healing, but perform different functions. An injury to the vascular system typically results in clotting due to the exposed red blood cells, which controls bleeding. Lymph fluid, however, lacks comparable coagulating properties. Moreover, the lymphatic circulatory system lacks the muscled walls of the vascular circulatory system. Stemming the outpouring of lymph, interstitial fluid and white cells involves compressing the lymphatic circulatory system through surrounding tissue swelling from an accumulation of edema and interstitial fluid. Unlike the quick response of coagulating red cells, lymphatic circulatory system closure tends to be slower and can take days.
Based on the involvement of the vascular and lymphatic circulatory systems in wound healing, influencing the performance of these circulatory systems can significantly improve wound healing. Wound closure can be achieved more quickly and infection risks can be reduced by controlling the factors affecting vascular and lymphatic circulation. For example, a profusion of blood flow in the wound site generally promotes re-epithelialization. The cells are also responsive to compression and tension. Properly applied and sequenced, compression and tension can promote healing.
The present invention addresses these wound healing factors by controlling and directing cellular functions based on contact and controlling such physical forces as shear, compression, tension and other physiological variables associated with the tissues and the fluids associated with the wound site and otherwise involved in the wound healing process.